1. Field of the Invention
The present invention relates to an optical disk and an apparatus for manufacturing the same. The term "optical disk" in this specification is intended to encompass a wide range of optical disks including an optical magnetic disk.
2. Description of the Prior Art
An optical disk has a clamping plate connected to the center of the same. The clamping plate is provided for rotating the optical disk and comprises a metal plate pressed into the shape of a disk. The clamping plate is connected to the center of the optical disk. In a MD(mini-disk), a clamping plate is not fixed to a disk but is connected to the disk with play therebetween. It is important that the clamping plate is connected precisely to the center of the disk. It is especially necessary that in a MD, the clamping plate is connected to the center of a disk with a precise clearance between the clamping plate and the disk. If the clearance is not precise, the connecting positions of the clamping plate and the disk cannot coincide with each other. The recording density of an optical disk is so high that the track pitch thereof is 1.6 .mu.m and the maximum line density is 0.6 .mu.m, and such an optical disk is required to be processed with high precision.
As shown in FIG. 1, a clamping plate 2 is connected to a disk 1 by crushing a part of, the disk. A heat press member 5 such as an ultrasonic horn or a heating horn is used for crushing a part of the disk 1. The lower end portion of the heat press member 5 has a cylindrical shape. The cylindrical lower end portion of the heat press member 5 is so formed as to heat press and deform the part of the disk 1 into an annular shape along the outer circumference of the clamping plate 2.
An optical disk is manufactured in the following manner.
1 A disk 1 is set horizontally on a press stand. PA0 2 A clamping plate 2 is set at the center of the disk 1. The disk 1 may be set on the press stand after the clamping plate 2 is mounted on the disk 1. PA0 3 By means of the heat press member 5, a part of the disk 1 is heat pressed into an annular shape along the outer circumference of the clamping plate 2. The heat pressed part of the disk 1 is inwardly projected as shown with broken lines in FIG. 1, so that the clamping plate 2 is connected to the disk 1. PA0 1 A disk on which a clamping plate is mounted at the center thereof is set on the press stand. PA0 2 The heat press member is urged onto the disk. A part of the disk around the clamping plate is heat pressed, and the heat pressed part of the disk is crushed. The crushed plastic material of the heat pressed part of the disk is protruded inwardly and the heat pressed part of the disk has a reduced inner diameter. With this heat pressed part having the reduced inner diameter, the clamping plate is connected to the disk so that it cannot be removed. The heat press member has grooves in the pressing surface thereof which is constructed to press the disk. The heat pressed part of the disk pressed by the pressing surface having such grooves is not deformed into an annular shape but is crushed into discontinuous segments. In the discontinued heat pressed part of the disk, warp is not locally concentrated even if the inner diameter of the heat pressed part is reduced. Therefore, unlike a conventional optical disk, an optical disk according to the present invention has no projection on the inside of the heat pressed part of the disk, and therefore can connect the clamping plate to the disk through a uniform width with high precision.
Optical disks manufactured in the above-mentioned method are advantageous in that they can be manufactured on a large scale and at low cost. This is because, in this method, the clamping plate can be connected to the disk by pressing a part of the disk and without using an adhesive agent, an adhesive tape or the like. However, an optical disk manufactured in such a method has a defect in that it is hard to heat press and deform the care of the disk into a precise annular shape so as to connect the clamping plate to the disk. The heat pressed part heated by the heat press member may thus have a projection such as shown by B in FIG. 2 on its inner periphery. The projection B will sometimes separate from the disk while the optical disk is used. A broken piece separated from the disk is apt to enter into a clearance between a reading head and the optical disk and cause a bit error. Further, such a broken piece will sometimes scratch the surface of the optical disk and thereby cause critical damage to the optical disk. Furthermore, a projection projected inwardly from the heat pressed part causes a reduction in the precision of the connection of the clamping plate with the disk. If the projection is in contact with the clamping plate, the clamping plate will not be uniformly connected to the disk.
The projection B is produced on the inside of the heat pressed part of the disk because the inner diameter of the heat pressed part is reduced through the heat pressing process. In other words, the part of the disk pressed by the heat press member is caused to protrude inwardly and is so deformed as to prevent the clamping plate from being removed. The inwardly protruded plastic part has a reduced inner diameter, and a force acts to radially urge the plastic material of the inner circumferential portion of the heat pressed part. This force causes partially protrusion of the plastics and thereby produces a projection.
A projection produced on the inside of the heat pressed part of the disk can be reduced by horizontally setting the disk on the press stand with high precision and precisely clamping the disk between the heat press member and the press stand. However, it is very difficult in practice to press with high precision each of the disks which are continuously fed one by one. It is especially difficult to control the feed position of each disk with high precision when the manufacturing efficiency is raised by reducing the tact time.
The present invention has been developed in order to solve the above-mentioned defects of the prior art. An important object of the present invention is to provide an optical disk for which the production of projection on the inside of the heat pressed part of the disk is reduced, the occurrence of bit errors caused by the projections is decreased and critical damage of the optical disk surface can be prevented. Another important object is provide an apparatus for manufacturing the optical disk.
The above and further objects and features of the present invention will more fully be apparent from the following detailed description with reference to the accompanying drawings.